Many hydrocarbons, including foods, lubricants and fuels, and substituted hydrocarbons, particularly polyolefin polymers and other plastics, are susceptible to oxidative and photo-initiated degradation. Conventionally, this degradation is retarded by utilizing additives which are termed "antioxidants" and "UV stabilizers", respectively. Polymers made from olefins are especially susceptible to oxidative degradation.
Low density polyethylene usually requires only a small amount of antioxidant and polystyrene likewise requires only a minimal amount of antioxidants. Nevertheless, high density polyethylene, high impact polystyrene, ABS terpolymers and polypropylene are included among commercial polymers which are exceedingly sensitive to oxidative degradation both while being processed and upon exposure to the environment.
It is generally accepted that polymer degradation by free radicals in the presence of oxygen involves chain scission followed by termination. Chain scission results in loss of molecular weight, increased melt flow, a decrease of polymeric roughness, some cross-linking and an eventual disintegration to powder.
Cross-linking results in an increase of molecular weight, embrittlement and a decrease in melt flow.
It is apparent that many polymers will have to be exposed to heat, both during processing and during their life in a particular use, and oxygen will also be unavoidable in these applications. Therefore, the adverse effect of degradation must be minimized in order to obtain a reasonable service life for the polymer material.
A great many compounds for stabilizing polyolefins have been developed with the object of retarding, inhibiting or otherwise modifying the degradation process. People disagree as to exactly how these stabilizers function.
There is general agreement that one class of materials prevents adverse degradation by oxidation by acting by a free radical scavenging mechanism. Examples of these are hindered phenols and amines.
Another type acts as peroxide decomposers which actually decompose the peroxides formed on the polymer backbone. Classes of compounds which appear to work in this manner are thioesters and phosphites.
Thioesters and alkyl aryl phosphates are generally used in combination with the hindered phenols and appear to interact with them to increase the overall stability in a synergistic manner although their primary function is apparently as peroxide decomposers.
Synergistic mixtures of alkylated phenols, or alkylidene-bis-alkylated phenols, and sulfur compounds have achieved active commercial success. For instance, dilaurylthiodipropionate has been used extensively commercially as the sulfur component of the synergistic mixture. Distearylthiodipropionate (DSTDP) is somewhat better as a synergist and is also gaining commercial acceptance.
Dimethylthiodipropionate is exceptionally useful as a synthetic intermediate to prepare a wide variety of such higher diesters. These higher diesters are conveniently prepared by transesterification. Thus, distearylthiodipropionate, dilaurylthiodipropionate, diabietylthiodipropionate, and the like, have all been conveniently prepared by transesterification from dimethylthiodipropionate.
Such higher diesters are not only extremely useful as a synergist antioxidant for various hydrocarbons, but they are also extremely useful as plasticizers/heat stabilizers for vinyl chloride polymers and synthetic rubbers.
They can be used as corrosion inhibitors. Not only are they stabilizers for polyolefins, but they are also antioxidants for food, cosmetics, lubricants, pharmaceuticals, and soaps.
Thus dimethylthiodipropionate is an extremely useful intermediate to prepare compounds of acknowledged utility for a great many well-defined, well-established end uses.
Nevertheless, the conventional synthesis of dimethylthiodipropionate suffers from a severe drawback that fairly large quantities of side products are produced. These side products adversely affect the color of the product and must be removed by expensive and difficult procedures such as vacuum distillation, solvent extraction or washing, etc.